The present invention relates to a device for sealing the slit in the housing of an encapsulated length or angle measuring instrument, through which passes a follower for mechanically connecting an object to be measured with a measuring component lcoated within the housing of the instrument.
Encapsulated length or angle measuring instruments are used to measure the relative position of two moveable objects. The housing is attached to the first object and a follower passes through a slit in the housing and connects the second object with a component of the measuring instrument. Typically the component is a scanning device which is caused by the follower to move along a measuring scale which is mounted to the instrument housing. Such encapsulated instruments are often used on machine tools to measure the relative position of the slide unit with respect to the machine bed.
Since these instruments are often used in environments subject to contamination from dust, oil, metal chips and the like, it is necessary to protect the highly sensitive measuring scale and scanning unit. For this reason the slit in the housing is often closed and sealed as well as possible. A measuring instrument of the type described above is disclosed in German Pat. No. 28 46 768 wherein the slit in the housing is closed by means of plastic or rubber sealing lips formed in a roof shape.
For this type of sealing mechanism to provide hermetic shielding of the housing, the sealing lips are typically made to lie close to the follower and are typically constructed of elastic material with a high restoring force. Problems with this type of sealing mechanism may arise because the high restoring force of the elastic material causes friction between the follower and the elastic material. The follower, subject to these frictional forces, may thereby be caused to undergo deformations in the measuring direction that are detrimental to the measuring accuracy of the instrument. Further problems with this type of sealing mechanism may arise with the seal at the fore and aft sections of the follower. Openings between the sealing elements often occur at these areas because of the torque, in the longitudinal direction, applied to the sealing lips by the passage of the follower through the lips. Although these openings may be reduced by constructing the follower with a sword-shaped cross-section, they nevertheless tend to expose the measuring instrument to possible contamination.